Gallium nitride materials are semiconductor compounds that have a relatively wide, direct band gap. These electronic transitions provide gallium nitride materials with a number of attractive properties, for instance the ability to withstand a high electrical field, to transmit signals at high frequency, and others. Gallium nitride materials are therefore being widely investigated in many microelectronic applications such as transistors, field emitters, and optoelectronic devices. Gallium nitride materials include gallium nitride (GaN) and its alloys such as aluminum gallium nitride (AlGaN), indium gallium nitride (InGaN), and aluminum indium gallium nitride (AlInGaN).
Most GaN epilayers are grown on foreign substrates such as sapphire (Al2O3), SiC or Si, because native GaN substrates are hard to make and as a result very expensive. These substrates have different structural and mechanical properties compared to the (In)(Al)GaN epilayers, for instance they comprise different thermal expansion coefficients or different lattice constants. This results in serious strain build-up in the GaN epilayer, which increases as the epilayer thickness increases.
In the prior art, a layered buffer structure is therefore introduced between the substrate and the active part of the device. This buffer structure accommodates as much as possible the effect of the difference between the properties of the substrate material and the materials that are used in the active part of the layers. Such differences may include but are not limited to, a difference in lattice constant, a difference in thermal expansion coefficient, a different crystal structure, a different band gap energy and resulting dielectric breakdown strength. This buffer layer ideally does not influence the properties of the active part or device, but could have minor functionality in the final device for instance as contacting layer, or alternatively as current blocking layer.
The active part of a layer stack is the part of the structure that directly determines the properties of the device that will be manufactured on the layer stack. For example, the active part of an AlGaN/GaN HEMT structure typically consists of a relatively thick (>100 nm) GaN channel layer with a thin (about 20 nm thick) AlGaN barrier layer on top. In such a HEMT, the threshold voltage, transconductance and for a part the on-state resistance are directly determined by the composition and thickness of an AlGaN barrier layer. In an LED for example, the composition and thickness of the quantum well and barriers determine the wavelength of the emitted light. The design and the choice of materials in the active part is optimized for the best device performance, and as little as possible dependent on constraints that are imposed by the choice of substrate or buffer structure.
In GaN on Si technology, almost always an additional nucleation layer is introduced between the substrate on the one hand and the buffer structure and active part on the other hand. This layer can be an AlN layer because the Gallium in AlGaN or GaN layers causes etch-back of the Si substrate. In some cases a dielectric layer is deposited on the substrate to mitigate this effect (e.g. SiC on Si, diamond in Si, etc.). The term “AlGaN” relates to a composition comprising Al, Ga and N in any stoichiometric/compositional ratio (AlxGa1-xN), which composition may vary in layer, e.g. from having no Ga at a bottom of the layer to having no Al at a top of the layer. A composition such as (In)AlGaN may further comprise Indium (In) an any suitable amount.
The buffer structure typically consists of a plurality of layers. In GaN on Si technology, the composition of the buffer layers will generally evolve from Al-rich layers in the vicinity of the AlN nucleation layer towards Ga-rich layers in the vicinity of the active part that typically comprises one or more GaN layers. The compositional variation from nucleation layer to active part can be done in various ways.
In case of GaN on silicium technology, the buffer structure should compensate for the tensile stress that is induced in the layer stack during the cool-down from the operating temperature during epitaxial deposition of the layer stack to room temperature. Typically, this is done by choosing the layers in the buffer structure such that the combination of these layers introduces compressive stress in the layers at the growth temperature. For instance, depositing a second AlGaN layer with lower Al-concentration on top of a first thick and relaxed AlGaN layer with higher Al-concentration will induce compressive stress because the larger lattice constant of the second layer will be compressed to match the smaller lattice constant of the first layer below.
For instance, in WO0213245 a use of a buffer structure is disclosed.
There exists an industrial need for improving buffer structures such that they compensate for the tensile stress that is induced in the layer stack during the cool-down from the operating temperature during epitaxial deposition of the layer stack to room temperature.